One can do the calculations or find recipies on the 'net but - it is just easy enough to prepare a 0.1M citric acid solution in water . prepare a 0.1M Nacitrate in water.

Using a pH meter (preferred) or pH paper add NaCitrate to the citric acid solution to achieve the desired pH

Thank you sir

 If you mix a 0,1M solution with another solution, you may reach the desired pH, but the final concentration wiil be smaller than 0,1M. So, if you need it to be 0,1M, you will not get it. If you are only interested in pH, will be OK,

Buffering capacity depends on the concentration of both acid and conjugated base (in this case citrate). Citric is a triprotic acid and for your pH you will have to get to cit3- (pKa3).

Perhaps you may use a 0,1M sodium citrate solution and add 1M HCl (perhaps higher) drop by drop until you get the desired pH. Thus you will minimise dilution of  the original solution.

Thank you miss

Silvana - if the two solutions mixed together are the same Molar concentration - the final resultant solution will be the same - no dilution (consider doing the calculations to convince your self).  Additionally, what some people forget is that that counterion in buffers can be terribly important - that is why unless you specifically state you want a citrate-HCL buffer solution you do not want to use HCL to adjust the pH of Na Citrate.  The dilution argument you state above applies to using NaOH to adjust the pH of a citric acid solution.  One compromise - make a stock citric acid solution of 0.5M final, adjust the pH to 7.0 recording the volume of acid used, then based on the total volume achieved dilute the mix to 0.1M - - that also would generate a 0.1M Na citrate buffer with a final pH of 7.0.  Best to check the pH post dilution - sometimes the activity coefficients of buffer components are such that the pH at a high concentration (e.g. 0.5) are not reflective at lower (0.1M).